1. Field of the Invention
This invention relates to a process and catalyst for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline, reformulation of gasoline for reduced emissions, and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline xe2x80x9coctanexe2x80x9d number. The catalytic reforming unit therefore must operate more efficiently at higher severity in order to meet these increasing needs for chemical aromatics and gasoline octane. This trend creates a need for more effective reforming processes and catalysts.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in bifunctional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. Commercialization has been slow and is limited in scope in light of special pretreating required to obtain the relatively high selectivity to aromatics that this technology features. Further, such catalysts yield a high proportion of benzene and toluene compared to the more desired xylenes. There is a need for further improvements in selectivity as well as stability of such dehydrocyclization catalysts.
The art discloses reforming with a broad range of catalysts containing large-pore zeolites and Group VIII metals. U.S. Pat. No. 4,104,320 (Bernard et al.) discloses dehydrocyclization with potassium-form L-zeolite charged with one or more dehydrogenating metals of Group VIII and another metal such as rhenium, iridium, tin or germanium. Bernard et al. teach that the other metal preferably is introduced at the same time as platinum or palladium and does not suggest such metals in the framework of the L-zeolite.
U.S. Pat. No. 4,990,710 (Dessau et al.) teaches dehydrogenation to yield aromatics using a tin-modified microporous crystalline silicate. U.S. Pat. No. 5,518,708 (Skeels et al.) teaches a molecular sieve having tin or chromium as framework tetrahedral oxide units, with zeolites Y and L being among the disclosed sieves. The use of such sieves is disclosed generally for a plethora of hydrocarbon-conversion, separation and oxidative combustion processes.
None of the above references discloses reforming with a catalyst containing a bound nonacidic large-pore molecular sieve containing framework tin and a platinum-group metal component.
It is an object of the present invention to provide a catalytic system and catalytic reforming process effective for the dehydrocyclization of paraffins and/or olefins with high catalyst selectivity and stability.
This invention is based on the discovery that a bound L-zeolite catalyst containing tin introduced via secondary synthesis and platinum results in substantial yield improvements in a catalytic reforming process.
A broad embodiment of the present invention is a reforming process, selective for dehydrocyclization of paraffins and/or olefins, using a catalyst comprising a bound nonacidic large-pore molecular sieve having a unit empirical formula on an anhydrous basis of mA:(SnwAlxSiy)O2; where A is at least one exchangeable cation selected from the group consisting of alkali and alkaline earth metals, xe2x80x9cmxe2x80x9d is the mole fraction of A and varies from about 0.01 to about 0.49, xe2x80x9cwxe2x80x9d is the mole fraction of tin and varies from about 0.01 to about 0.49, xe2x80x9cXxe2x80x9d is the mole fraction of aluminum and varies from about 0.01 to about 0.49, and xe2x80x9cyxe2x80x9d is the mole fraction of silicon and varies from about 0.50 to about 0.98. The tin preferably is introduced into the sieve via secondary synthesis, especially via a solution of a fluoro salt of tin. The molecular sieve is bound using an inorganic-oxide binder which preferably comprises one or both of silica and alumina. The catalyst comprises at least one platinum-group metal component, preferably comprising platinum. Optimally, the molecular sieve comprises potassium-form L-zeolite.
The reforming process converts a hydrocarbon feedstock with high selectivity to obtain an aromatics-rich effluent. Operating conditions comprising low operating pressures, optimally between about 100 and 300 kPa, are used to advantage. The process is particularly effective in aromatization of paraffins and/or olefins to yield aromatics. Preferably individual paraffin and/or olefin isomers are aromatized to corresponding aromatic isomers of the same carbon number in high yield.
These as well as other objects and embodiments will become apparent from the detailed description of the invention.